The invention relates to a method of regenerating a used catalyst with fluidized bed heat exchange and an apparatus for carrying out the method. More particularly, the method can be applied to the regeneration of catalysts contaminated with coke residues after reaction with a hydrocarbon batch. It may relate to hydroprocessing, hydrocracking or catalytic cracking, reforming catalysts or even any contact mass used for example in thermal cracking processes.
Purely by way of illustration, the process can be applied to the regeneration of used catalyst resulting from a catalytic cracking process conducted in a fluidized bed and involving heavy oil batches having a high Conradson carbon level, such as an atmospheric residue, a vacuum residue, a deasphalted residue, such residues being capable of being hydroprocessed.
The method of the invention is particularly applicable to the monitoring of temperature.
Catalytic cracking processes convert the hydrocarbon carbon charges to lighter products such as fuels. Initially, the charges were relatively light, diesel fuel for example, and in order to obtain maximum conversion efficiency with highly active zeolitic catalysts, it was necessary to remove the maximum of the coke which had become deposited on these catalysts, reducing their effectiveness, during a regeneration stage at a temperature of between 520.degree. and 800.degree. C.
The urgent demand for fuels is motivating refiners to turn their attention to increasingly heavier charges, containing hydrocarbons with a high boiling point, for example with a boiling point above 550.degree. C. and having a high Conradson carbon level or a substantial concentration of metals. Heavy hydrocarbons and coke can then become deposited on the catalyst in a substantial quantity during the catalytic cracking phase and regeneration by combustion may result in a considerable release of heat which may damage the apparatus and deactivate the catalyst, particularly in the event of prolonged exposure to temperatures higher than 800.degree. C. It then becomes vital to monitor catalyst regeneration. This problem occurs particularly when it is desired to apply to a long-standing, technology dealing essentially with conventional hydrocarbon charges to a method which involves much heavier charges.
One of the objects of the invention therefore is to propose a regeneration process and an apparatus comprising the monitoring of cooling of the catalyst in a catalytic cracking unit with a view to processing heavy charges.
Another object of the invention is to provide for much greater flexibility of use of the apparatus.
The prior art is illustrated by the following patents:
U.S. Pat. No. 4,614,724 discloses an apparatus embodying a regenerator, the regeneration temperature of which is controlled by an external heat exchanger in which the flow passes downwardly through a nest of tubes.
The cooled catalyst is recycled to the generator via a duct through which catalyst ascends in a fluidized state into the dense bed of the regenerator. The catalyst in the exchanger is maintained in dense bed by a fluidizing gas flowing counter-current to the direction of flow of the catalyst and the fluidizing gas is either entrained with it when the rate of flow is very low or is discharged through the catalyst line. This counter-current circulation of the gas disturbs the flow of catalyst in the inlet connector and in the exchanger and the heat exchange efficiency is therefore not at its maximum.
U.S. Pat. No. 4,434,245 describes a two-level regenerator comprising an external exchanger with lateral intake of hot catalyst emanating from the upper level which is a storage zone.
The cooled catalyst is recycled via a duct receiving the regenerating air and the used catalyst, into a zone corresponding to the lower level where combustion takes place. Therefore, operation of the regenerator and of the exchanger is closely linked since the return of cooled catalyst to the regenerator is a function of the rate of flow of fluidization of the air used for regeneration and which circulates within the said duct. Furthermore, this patent discloses a connector above the exchanger and which discharges into the dense bed of the exchanger so that the release of gas and fumes can only take place partially, taking into account the presence of the catalyst in this connector. There may then be a phenomenon of circulation of catalyst with a return flow through the top (backmixing). Clearance of gas is all the poorer since the nest of exchange tubes reaches as far as the top end of the exchanger. Mixing is not necessarily homogeneous and there is therefore an upper zone where the catalyst stagnates and where it is only poorly renewed, so that heat exchange is diminished.
U.S. Pat. No. 4,923,834 describes a "backmixing" process in which an upper connector discharging into the inlet duct of the exchanger containing catalyst which circulates in a dense bed, makes it possible for catalyst to return from the exchanger into the storage chamber of the regenerator. Here, therefore, there is cooling by "backmixing" and not a solution to a problem of evacuation from a heat exchanger of fumes and fluidizing air which would make it possible to maximise heart exchange.